Thermally foamable microspheres, also called thermally expansible microcapsules, are now put into practical use in various fields, for instance, in the form of fillers for paints or plastics added for weight-reduction purposes, let alone foaming inks. Thermally foamable microspheres have usually volatile liquid foaming agents microcapsulated with polymers. Generally, the thermally foamable microspheres may be produced by the suspension polymerization in an aqueous medium of a polymerizable mixture that contains at least a foaming agent and a polymerizable monomer. As the polymerization reaction goes on, a shell is formed by the resulting polymer, yielding a thermally foamable microsphere having a structure wherein the foaming agent is wrapped up and encapsulated in the shell.
As applications of thermally foamable microspheres spread and their performance demanded in individual applications becomes high, the level of performance demanded for them are going up.
One of the current requirements for thermally foamable microspheres is that on and after foaming by heating, there be little or no agglomeration due to fusion among foamed particles. Another requirement for these microspheres is that they exhibit sharp foaming start behavior even upon subject to thermal hysteresis at high temperatures on drying, etc.
Thermally foamable microspheres are not only incorporated in inks, paints, plastics, etc. while they remain unfoamed but are also sometimes used in foamed states. Foamed particles, because of being hollow plastic balloons, are so much reduced in weight that they can be used as a filler material for paints as an example, thereby reducing the weight of the application member to be coated. However, agglomeration among foamed particles would make mixing with substrate materials for paints, etc. difficult and, at worst, cause destruction of the foamed particles during mixing.
One possible approach to prevention of agglomeration among foamed particles is to cover unfoamed thermally foamable microspheres with inorganic fine particles. However, it is very difficult to adhere the inorganic fine particles uniformly to the surface of a thermally foamable microsphere and place the amount of adherence under strict control. A failure in the uniform adherence of the inorganic fine particles to the surface of the thermally foamable microsphere would render uniform mixing difficult. Too little inorganic fine particles would make sufficient prevention of fusion upon foaming by heating impossible. Too much would render sufficient foaming difficult and, at worst, impossible.
Yet another requirement for thermally foamable microspheres is to ensure sharp foaming thereby yielding foamed particles of uniform shape and size. To this end, selection of polymerizable monomers and improvements in suspension polymerization conditions are now proposed (e.g., JP-A 11-209504). Only by means of adherence of inorganic fine particles to the surface of a thermally foamable microsphere, however, it is still difficult to gain control of foamability, and so it is impossible to achieve such good properties.
For the production of thermally foamable microspheres by suspension polymerization in an aqueous dispersion medium, it is known that use of an aqueous dispersion medium containing colloidal silica or other inorganic fine particles as a dispersion stabilizer permits the inorganic fine particles to adhere to the surfaces of the resulting thermally foamable microspheres. Because of a weak force of adherence of the inorganic fine particles to the thermally foamable microspheres, however, the inorganic fine particles in an amount enough to prevent fusion among foamed particles cannot adhere uniformly and firmly to the surfaces of the thermally foamable microspheres. In this case, when the reaction mixture is filtered and washed in a recovery step after the completion of polymerization, nearly all of the inorganic fine particles used as the dispersion stabilizer will remain contained in filtrates.
Even though the inorganic fine particles adhere to the surfaces of thermally foamable microspheres, they will be readily released off in washing and subsequent steps, and it is difficult to gain strict control of the amount of adherence of the inorganic fine particles. In addition, use of colloidal silica or the like as the dispersion stabilizer causes filtrates to become cloudy. The cloudy filtrates cannot be disposed without charging a flocculant therein for sedimentation, followed by centrifugal removal of cloudy matter. Release of the inorganic fine particles off the surfaces of the thermally formable microspheres in the subsequent treatment steps will cause dusting that may otherwise render working environments worse.
By increasing the amount of the dispersion stabilizer such as colloidal silica, it is possible to increase the amount of adherence of inorganic fine particles to the surfaces of thermally foamable micro-spheres. However, clouding problems with filtrates and problems due to the release of inorganic fine particles in the washing and subsequent steps remain still unsolved. In addition, the thus increased amount of adherence of inorganic fine particles to the surfaces of thermally foamable microspheres incurs another problem that the average particle diameter of the obtained thermally foamable microspheres becomes small whereas the particle diameter distribution thereof becomes wide.
On the other hand, there is a growing demand for thermally foamable microspheres exhibiting sharp foaming start behavior. In some cases, thermally foamable microspheres are dispersed in an aqueous dispersion medium such as an aqueous emulsion for use depending on applications. Using a coater or the like, an aqueous dispersion containing thermally foamable microspheres is coated on a primer material, and then dried and foamed by heating. Often in this case, drying is carried out by means of the spraying of hot air at elevated temperature for the purpose of cutting off the drying time. Upon subject to thermal hysteresis at high temperature during drying, some portions of conventional thermally foamable microspheres are foamed even at temperatures lower than the foaming temperature and, consequently, they exhibit generally broad foaming behavior. This in turn causes damage to the surface properties and smoothness of a foamed particle layer, giving rise to problems on practical use.
Some publications (e.g., Japanese Patent Application No. 2000-131859 corresponding to JP-A 2002-12693 and WO 01/83636) propose that the foaming behavior of thermally foamable microspheres can be controlled by way of control of the modulus of elasticity of a shell resin with respect to temperature and control of the vapor pressure vs. temperature of a foaming agent encapsulated in the shell. Only with such a prior art, however, it is still difficult to obtain thermally foamable microspheres exhibiting sharp foaming behavior even via thermal hysteresis at high temperature during drying.
Thermally foamable microspheres having improved adhesion to other materials are also in demand. In most cases, thermally foamable microspheres are incorporated in substrate materials for inks, paints, plastics, etc. in unfoamed states, and then heated for foaming. However, when foamed particles are poor in adhesion to such substrate materials, various inconveniences are likely to occur in connection with appearances and physical properties. In this case, how to modify shells is of importance. If the adhesion of the shells of thermally foamable microspheres in general and the surface adhesion in particular are improved, adhesion to various substrate materials can then be improved.
Furthermore, if the adhesion of the shells of thermally foamable microspheres can be enhanced, it is then possible to provide uniform and firm adherence of inorganic fine particles thereto and gain strict control of the amount of adherence. This in turn will allow the foaming behavior of thermally foamable microspheres to be precisely controlled and designed. As a result, there will be provided new technical means for preparing hybrid hollow microspheres or functional additives comprising resin particles coated with inorganic fine particles. However, never until now is anything proposed about technical means for improving the adhesion of shells of thermally foamable microspheres.